What organization is responsible for port security, immigration control, and navigational aid safety?

The Coast Guard is the organization responsible for port security, immigration control, and navigational aid safety. This branch of the U.S. Armed Forces operates under the Department of Homeland Security during peacetime and serves a variety of critical roles, including ensuring the safety and security of U.S. waterways, enforcing maritime laws, and conducting search and rescue operations.

The Coast Guard's port security responsibilities involve protecting ports and maritime infrastructure from threats, which includes monitoring vessels and conducting inspections. Immigration control is another key function, as the Coast Guard works to enforce laws regarding the entry of individuals into the country via maritime routes. Additionally, the Coast Guard oversees navigational aids, such as buoys and lights, which help guide ships safely through waterways, ensuring that navigation is both effective and safe for all maritime traffic.

While the Marines and Navy also play important roles in national security, their primary focus is different. The Merchant Marine primarily deals with commercial shipping and logistics rather than security and immigration control. Hence, the Coast Guard stands out as the correct answer for the responsibilities outlined in the question.